Process for manufacturing a circuit with filled holes

ABSTRACT

Circuit boards are manufactured by forming a substrate with a dielectric surface, laminating a metal foil to the substrate, and forming holes in the substrate through the foil. A filler material with an organic base may be filled with electroconductive particles or dielectric thermoconductive particles. The filler material is deposited into the holes and is heated to at least partially cure it. The surface of the filler material is seeded and electrolessly plated to form a conductive coating on the metal foil and the filler material. The coating is then patterned to form a wiring layer. A second set of holes may be formed in the circuitized substrate after the hole filling step, which are also electrolessly plated.

This is a Division of Ser. No. 08/672,292 filed on Jun. 28, 1996.

FIELD OF THE INVENTION

This invention relates to high density computer systems using circuitboard assemblies and manufacture of circuit board assemblies in whichsurface mount components and pin in hole components are attached tocircuitized substrates by soldering component terminals into platedthrough holes and to connection pads on the substrate. More particularlythis invention relates to methods for forming electrically conductivevias between buried wiring layers; forming multiple very fine externalwiring layers; and organic materials made conductive by filling withconductive particles.

BACKGROUND

The following background is for convenience of those skilled in the artand for incorporating the listed citations by reference. The followingbackground information is not an assertion that a search has been made,or that the following citations are analogous art, or that any of thefollowing citations are pertinent or the only pertinent art that exists,or that any of the following citations are prior art.

The continued introduction of very high I/O and very high densitysurface mount components especially 0.2-0.4 mm gull wing leadedcomponents, 40 mil ball grid array BGA modules, as well as the directconnection flip chips to circuit boards, has resulted in a need for veryhigh density conductor fan out at these components. At the same time,decrease in the size of plated through holes PTHs which interconnectbetween wiring layers, has not kept up with these requirements for fanout. PTHs require substantial surface area which can not be easilyreduced because seeding and plating require circulation of fluids in theholes. Reducing the size of connections between wiring layers has becomecritical for continued increase in circuit board density.

For such high density surface mount components, solder volumes are acritical process variable, but when components are attached to PTHs eventhose filled with solder, the solder volume between the terminals of thecomponents and the PTHs can not easily be controlled.

Those skilled in the art are directed to the following references. U.S.Pat. No. 4,967,314 to Higgins, III suggests filling via interconnectholes with a conductive epoxy. U.S. Pat. No. 3,163,588 to Shorttsuggests stripable frisket, seeding and electroplating. Face Protectionof Printed Circuit Boards by McDermott in IBM Technical DisclosureBulletin Vol. 11 No. 7 December 1968 describes peelable coverings andpressing resin into plated through holes. Printed Circuit Base byMarshall in IBM TDB Vol. 10, No. 5, October 1967, describes asensitizing material. U.S. Pat. No. 4,590,539 to Sanjana disclosesepoxies, fillers, curing agents, and catalysts. U.S. Pat. No. 4,791,248to Oldenettel suggests peel apart coverings, filling holes with resin,planing off resin and nubs. U.S. Pat. No. 4,893,440 to Shirahatadiscloses buried vias and electroconductive organic based paste. U.S.Pat. No. 4,964,948 to Reed suggests methods for seeding a substrate forelectroplating. U.S. Pat. Nos. 4,991,060 and 5,028,743 to Kawakamisuggests filling through holes with electroconductive paste and buriedvias. U.S. Pat. No. 5,065,227 to Frankeny suggests electricallyconductive paste filling a via hole. U.S. Pat. No. 5,243,142 to Ishikawadiscloses hole fill. U.S. Pat. No. 5,271,150 to Isasaka disclosesmanufacturing methods for multi-layer ceramic substrates includingfilling holes punched in green sheets with conductive paste. U.S. Pat.No. 5,319,159 to Watanabe suggests method of manufacturing a doublesided printed wiring boards with resin filled PTHs. Japanese application2-045998 suggests filling through holes with electroconductivethermosetting paste. Japanese application 2-184626 to Honda suggestsusing a novolac epoxy resin such as cresol novolac epoxy resin for acircuit board. U.S. Pat. No. 5,346,750 to Hatakeyama suggests a methodto prevent bleed out of paste from a filled via. U.S. Pat. Nos.4,354,895 to Ellis, 5,057,372 to Imfeld, and 5,262,247 to Kajiwarasuggests a metal foil with a peel apart protective layer. U.S. Pat. Nos.5,200,026 to Okabe and 5,266,446 to Chang suggest processes for formingthin film structures on substrates. U.S. Pat. Nos. 4,940,651 to Brown,5,026,624 to Day, 5,070,002 to Leech, 5,300,402 to Card, 5,427,895 toMagnuson, and 5,439,779 to Day discuss photoresists. U.S. Pat. Nos.4,127,699 to Aumiller, 4,210,704 to Chandross, 4,731,503 to Kitanishi,4,747,968 to Gilleo, 4,822,523 to Prud'Homme, 4,880,570 to Sanborn,4,904,414 to Peltz, 4,999,136 to Su, 5,082,595 to Glackin, 5,220,724 toGerstner, and 5,463,190 to Carson suggest various electricallyisotropically conductive organic materials. New Avenue for Microvias inElectronic Engineering Times Mar. 18, 1996, p. 68 reports that ProlinxLabs Corp of San Jose Calif. has developed an additive technology forblind and buried vias filled with conductive material. The proceedingcitations are hereby incorporated in whole by reference.

SUMMARY OF THE INVENTION

In the invention of Applicants, holes in a first substrate structure arefilled with an organic based conductive material and additionalsubstrate layers are laminated to the substrate without causing theorganic material to bleed out between the first substrate and theadditional substrates during lamination. Also, metal may be plated overthe organic material prior to laminating additional substrates onto thefirst substrate. The organic material may be filled withthermoconductive dielectric particles for better thermal performance orfilled with electroconductive particles to provide conductive holes.Conductive vias in the additional substrates may be plated to connect tothe conductive material in the filled holes. A cresol-novolac epoxyprecursor may be filled with 70-80% by weight electroconductiveparticles such as copper and/or silver powder and cured and maysubsequently be plated with copper. This allows conductive vias such asPTHs filled with electroconductive or thermoconductive material or holesfilled with electroconductive material to be buried in the substrate andavoids excessive surface area from being utilized for vias betweeninternal wiring layers. Also, the exterior substrate may be providedwith photo-vias which require much less surface area than PTH's. Theinvention allows decreased wiring lengths between components so that acomputer system utilizing such circuit boards will operate at a higherspeed.

The invention includes a process for producing the circuit boards of theinvention. In the process a peel apart structure including a copper foiland removable film is positioned with the copper foil in contact withthe substrate and is laminated to a substrate. Then holes are formed inthe substrate through the peel apart structure. Also, a sacrificialcarrier is coated with an organic resin filled with conductive particlesand then partially cured to form a carrier structure. The partiallycured resin is positioned between the removable film and the sacrificialcarrier and heat and pressure is applied to force the partially curedfiller material into the holes with the peel apart structure acting as amask. Finally, the removable film, the sacrificial carrier, and theconductive material remaining therebetween are peeled away leaving thecopper foil laminated to the substrate.

The surface of the filler material is flattened by abrasion to the samelevel as the copper foil and the surface of the filler material isplated with copper. The copper foil is patterned to form a wiring layerthen a dielectric photoresist is laminated to the wiring layer. Sincethe filler material is partially cured and covered by electroplatedcopper it does not bleed from the holes in between the wiring layer andthe photoresist. Conductive photo vias which are connected to the wiringlayer, are formed through the photoresist and copper is deposited on thephotoresist and patterned to form another wiring layer over thephotoresist and also connected to the conductive vias.

The invention also includes the structures that result from the processincluding the substrate with the peel apart structure and carrierstructure laminated thereto; the substrate with holes filled with thefiller material; and the substrate with the photoresist laminatedthereto with conductive vias and exterior wiring layer over thephotoresist. The invention also includes an improved circuit boardassembly including surface mount components placed at a higher densityto allow faster circuit board speeds. Furthermore, the inventionincludes a computer system which operates faster due to the shortersignal flight times which result from the higher wiring densities of theinvention.

Other features and advantages of this invention will become apparentfrom the following detailed description of the presently preferredembodiments of the invention illustrated by these drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(a)-1(j) is a flow diagram illustrating a specific embodiment ofthe process of the invention.

FIGS. 2(a)-2(c) is another flow diagram illustrating an alternativespecific embodiment of the invention.

FIG. 3 schematically shows a portion of a substrate of the inventionwith holes through a peel apart structure and a transfer structure withfiller material before lamination.

FIG. 4 schematically shows the portion of the substrate of FIG. 3 withholes substantially filled with electroconductive material afterlamination and peeling apart.

FIG. 5 schematically shows the portion of the substrate of FIG. 3 afteradditional layers have been formed.

FIG. 6 schematically shows circuit assemblies of the invention assembledinto an enclosure with a power supply to form the computer system of theinvention.

The following detailed description discusses specific examples andembodiments of the inventions including the best mode for utilizing theinventions, in such detail that anyone skilled in the art may utilizethe invention without an undo amount of experimentation.

BEST MODE FOR CARRYING OUT THE INVENTION

In step 100 of FIG. 1(a) a circuit board substrate is formed. Thesubstrate may be a ceramic substrate (e.g. alumina, or beryllia); or ametal substrate (e.g. Cu, Al, Invar, Covar, or Cu--Invar--Cu) coveredwith dielectric material (e.g. polyimide, or epoxy); or an organicsubstrate (e.g. epoxy) preferably filled with axially stiff fibers(fiberglass or polyamide fibers) or a flexible substrate of dielectricpolymer films (e.g. polyimide) and metal foils (e.g. copper). Forexample in FIG. 3 circuit board substrate 302 includes two buried metalwiring layers 304, 306 (power and ground planes) and three dielectriclayers 308, 310, 312. The dielectric layers may be ceramic or organicmaterial.

In step 102 of FIG. 1(a) metal foil is laminated to a removable coveringto form a peel apart structure. Preferably the metal of the foil iscopper and the removable covering is a dry polyimide film or a secondmetal foil. A dry adhesive film may be provided between the metal foiland the removable foil. The adhesion between the adhesive film and theremovable layer should be greater than the adhesion between the adhesivefilm and the metal foil or the adhesive film will have to be strippedoff the foil after peeling. Also, for this process a layer ofphotoresist may be provided between the removable layer and the metalfoil. After the removable layer is peeled off, the copper layer will beready for photolithography. The adhesion between the removable coveringshould be sufficient to prevent separation during normal handling, butshould be sufficiently low to prevent tearing the metal film orremovable film during peeling and prevent delamination of the copperfilm from a substrate during peeling.

A peel apart copper structure is available from Gould.

In step 104 the peel apart structure is positioned adjacent the circuitboard with the copper foil against the circuit board substrate. In step106 the peel apart structure is laminated to the circuit boardsubstrate. The copper foil is laminated to a dielectric surface of thecircuit board substrate. These steps may be performed by feeding theboards together with the peel apart structure between two heatedrollers. FIG. 3, shows a peel apart structure laminated to each side ofthe substrate. On the top of the substrate a metal foil 314 is laminatedto dielectric layer 308 and removable layer 316 is laminated to metalfoil 314. On the bottom of the substrate metal foil 318 is laminated todielectric layer 312 and removable layer 320 is laminated to foil 318with intermediate layer 322 between the metal foil and removable film.The intermediate layer may be a photoresist which is left on the foilafter peeling to provide for photolithographically patterning the foil.Alternately the intermediate layer may be an adhesive for laminationwhich is preferably peeled off with the removable layer.

In step 108 holes are drilled into the substrate through the peel apartstructure. As shown in FIG. 3, hole 326 extends through the substrateand through both peel apart structures laminated to the surfaces of thesubstrate. Holes 328, 330, 332 are blind holes or cavities thatpreferably extend through the peel apart structure and into thesubstrate to buried wiring layers as shown. The holes are 4 to 24 milsin size, preferably 8 to 16 mils, most preferably about 12 mils. Fillingholes as small as 2 mils may be possible if the substrate is thin andthe process is optimized. The through holes may be plated at this timeif a dielectric filler material is to be used to provide filled PTHs.

In step 110 of FIG. 1(b) an organic filler material is prepared. Theorganic material may be filled with electroconductive particles to forman organic based conductive material. Alternately, the organic materialmay be filled with thermoconductive dielectric particles to increasethermal performance. The organic material may be a thermoset or athermoplastic resin and preferably is an adhesive (epoxy or silicone)and more preferably 20% to 30% by weight phenol cured cresol-novolacresin. Methyl ethyl ketone solvent may be added to the organic materialto achieve suitable viscosity for coating. A tertiary amine catalyst isadded for curing. Electroconductive particles may include metal orcarbon and may include silver flakes or silver particles but arepreferably copper particles. Alternatively the particles may include atransient liquid phase TLP particle system. Preferably the coefficientof thermal expansion of the filler material matches the coefficient ofthermal expansion of the substrate in the direction of the through hole.Preferably conductive particles have a maximum size of 6 microns.

TLP systems when initially heated form a molten eutectic alloy portionwhich immediately resolidifies because the molten alloy portion is incontact with a supply on one of the constituent elements of the alloywhich element dissolves into the molten alloy until the molten alloy isno longer eutectic and melting temperature is increased. TLP particlesystems for organic based electroconductive materials are based onsolder alloys in which the elements of the solder alloys are not yetalloyed. For example binary eutectic solders are alloys of a first metaland a second metal and a binary TLP particle system may containparticles of the first metal coated with the second metal or the firstmetal coated with the eutectic alloy of two metals or particles of thefirst metal mixed with particles of a eutectic alloy of the two metalsor a mixture of coated particles and uncoated particles. Trinary andquatrinary solders can also be used. TLP particle systems when heatedinitially form a melted eutectic portion at the particle surface toconnect the particles together, but the core of the particles includenon-eutectic amounts of one of the metal constituents of the alloy sothat as more of the particle dissolves into the molten surface themolten alloy becomes non-eutectic and the melting temperature of themolten portion rises until it solidified even at constant temperature.For example lead particles coated by a tin layer having 3% of the massof the particle when heated to 180° C., will initially form a moltencoating of eutectic 63/37% Sn/Pb alloy, allowing adjoining particles toconnect together. As more of the Pb core of the particle dissolves intothe molten portion the lead content of the molten alloy increases andmelting temperature increases and the molten portion solidifies. Tinparticles coated with lead will react similarly and mixtures of pure Pbparticles and eutectic Pb/Sn particles will also react similarly wheredissimilar particles meet.

In step 112 a sacrificial carrier is coated with the filler material toform a transfer structure and in step 114 the filler material is heateduntil partially cured. The sacrificial carrier may be a metal foil or anorganic film such as polyimide. For thermosets containing solventspartial curing comprises heating for driving out the volatile solventsand for epoxies partial curing includes heating to form long polymerchains until a B-stage is reached. In step 116 the transfer structure ispositioned adjacent to the substrate with the filler material againstthe removable layer of the peel apart structure. FIG. 3 shows a transferstructure on each side of substrate 302. On the top surface of thesubstrates filler material 340 is deposited on sacrificial layer 342 andthe transfer structure is positioned with the filler material againstremovable layer 316 of the peel apart structure. On the bottom surfaceof the substrate, filler material 344 is deposited on sacrificial layer346 and the transfer structure is positioned with filler material 344against removable layer 320 of the peel apart structure.

In step 118 the transfer structure is laminated to the substrate withsufficient heat and pressure to force the filler material to fill theholes. For electroconductive filler material the holes must be filledsufficiently to provide electrical connection between the ends of thefilled holes. A lamination press is preferred for this lamination stepbecause of the high pressures and relatively high thickness of thetransfer structure.

In step 120 of FIG. 1(c) the removable layer along with the sacrificiallayer and filler material remaining between the layers is peeled off thesubstrate. FIG. 4 shows the resulting structure with nubs 350 of fillermaterial extending from the holes. In this view the filler material 352in through hole 326 is continuous from metal layer 314 to metal layer318 to provide electrical interconnection. Any voids 354 aresufficiently small that electrical conduction is not significantlyaffected.

In step 122 the nubs are abraded off even with the external surface thecopper foil. This is shown in FIG. 4 where nubs 350 are shown extendingabove the level of the copper foil 314 and where any filler materialextending beyond foil layer 318 has been removed by abrasion (sanding).Alternately the material may be removed by planing.

If only one surface wiring layer is desired on each side of thesubstrate and unfilled plated through holes PTHs are desired for exampleto attach pin in hole PIH components, then holes should be drilledthrough the substrate and seeded for electroless metal plating beforestep 124.

In step 124 metal is deposited on the filler material and metal foil toform a continuous layer of metal on each of the major surfaces of thesubstrate. For electroconductive filler material the conductive materialmay be prepared for plating by exposing conductive particles on thesurface of the conductive material. The deposition may includesensitizing or seeding the surface of the filler material andelectrolessly plating to form a thin coating of copper on the surface ofthe filler material. Preferably the deposition includes electroplatingof copper onto the filler material on both major surfaces. This stepseals the filler material into the holes and for electroconductivematerial provides a better electrical connection than the connectionbetween the conductive material and the walls of the holes through themetal film. Preferably the plating continues until 0.5 to 2 mils ofcopper are deposited on the walls of the plated through holes and about0.2 to 1.0 mils are deposited on the surface of the filler material inthe holes. FIG. 5 shows the metal plated over the filler material forexample at 502.

The metal surface may be vapor blasted and/or treated in a chloritingbath to increase adhesion to a photoresist. Also, microetching or pumicewashing may be used to improve adhesion.

In step 130 of FIG. 1(d), a layer of first photoresist is formed overthe continuous layer of metal. A liquid precursor may be spun on thesurface and cured or more preferably a dry film photoresist 0.1 to 4mils thick is used. In step 132 the photoresist is exposed to a patternof electromagnetic radiation or a particle beam. The radiation may beproduced in a pattern using a laser or a source of visible light, UVlight, or X-ray which may be directed through a mask to form a pattern.The type of radiation or particle beam depends on equipment availabilityand the chemistry of the photoresist. In step 134 the photoresist isdeveloped to form a first pattern of photoresist. The pattern coversportions of the metal layer which will form a wiring layer on thesurface of the substrate. Other portions of the continuous metal layerare exposed and in step 136, the exposed portions are etched away toform a first wiring layer (signal layer). For copper the preferredetchant is cupric chloride but other etchants used for etching copper incircuit board manufacture may be used. The first wiring layer 504 and506 is shown in FIG. 5. Then in step 138 of FIG. 1(d) the firstphotoresist is stripped away for example by rinsing with deionizedwater. The photoresist may be a positive resist in which case exposedportions become softened and are rinsed away to form the photoresistpattern and after etching the remaining photoresist is exposed toradiation and rinsed away to strip the photoresist off the metal layer.Alternatively, for negative photoresists the exposed portions becomehardened and the unexposed portions are rinsed away. Then the firstphotoresist pattern is removed using a solvent or etchant.

Preferably in wiring layers 504, 506 a narrow metal land surrounds theexterior ends of the holes filled with filler material as at 507 in FIG.5.

If only one external wiring layer is desired on each side of thesubstrate then the circuit board may be completed by covering each sidewith a layer of solder resist and forming windows in the solder resistfor surface mount connection pads. Otherwise if more layers are desiredthen processing continues as follows.

The following steps 140-166 may be performed sequentially multiple timesas desired, to provide multiple wiring layers on each of the surfaces ofthe substrate.

In step 140 in FIG. 1(e) a layer of second photoresist is formed overthe wiring layer. Again, a dry film photoresist is preferred. The secondphotoresist can be the same material or a different material than thefirst photoresist and either a positive or negative photoresist.Preferably the second photoresist is a photoimagable and can be treatedfor use as a permanent dielectric layer (photoimagable dielectric) thatcan withstand solder reflow temperatures.

If desired, additional holes filled with filler material such as hole516 in FIG. 5, may be formed some time between steps 140 and 150 bylaminating a peelable layer over the photoimagable dielectric; drillingholes through the peelable layer, photoimagable dielectric, and into thesubstrate; plating the holes with metal if desired; laminating anothertransfer structure to the peelable layer with sufficient heat andpressure to force the filler material into the holes; and peeling awaythe peelable layer and transfer structure.

In step 142 the photoimagable dielectric is exposed to a pattern ofelectromagnetic radiation or particle beam, and in step 144 thephotoimagable dielectric is developed to form a corresponding pattern ofphotoresist. Preferably as shown in FIG. 5, the pattern of photoresistlayers 508, 510 consist only of via holes such as at 512, 514 thatextend through the photoresist over pads or conductors of the firstwiring layer. In step 146 the photoresist is treated to make itpermanent for example by baking a positive photoresist so that it is notaffected by subsequent exposure to light. If components need to beattached by solder reflow the permanent photoresist layer must becapable of withstanding those temperatures. Other attachment methodssuch as ECA component attach require much lower cure temperatures. Thisstep may be required for some negative photoresists so that subsequentplating, etching, developing steps do not affect the photoresist. Othernegative photoresists may not require this step.

In step 150 in FIG. 1(f) a third layer of photoresist is formed over thephotoimagable dielectric, and in step 152 the third photoresist isexposed to a pattern of electromagnetic radiation or particle beam.Again this is commonly done by directing light (visible, or UV) from asource through a mask. In step 154 the third photoresist is developed toform a third photoresist pattern.

The following steps 156 and 158 are performed when PIH components arerequired when forming the last wiring layer on each surface of thesubstrate. For example in FIG. 5 three external wiring layers areprovided and PTHs 518 are only provided when forming the final wiringlayer.

In step 156 of FIG. 1(f) holes are formed through the substrate toprovide interconnection between wiring layers and/or PTHs for PIHcomponents. The holes may be formed by laser drilling, punching, or bymechanical drilling using a drill bit. In step 158 the holes are treatedto remove debris and improve electrical connection. For holesmechanically formed using a drill bit, the holes should be deburred andchemically cleaned to remove smear from internal wiring layers forelectrical connection thereto.

In step 160 of FIG. 1(g) the surface of the photoimagable dielectric aswell as the walls of the photo-vias and any holes for PIH components,are seeded for electroless metal plating. In step 162 a thin coat ofmetal is formed on the seeded surfaces by electroless plating. In step164 a thicker metal coating is formed by electroplating. Preferably thethicker coating is copper with a thickness of 0.2 to 4 oz of Cu persquare foot, more preferably about 1 oz (0.5-2 oz) per square foot.Preferably the copper is at least 1 mil thick in any plated throughholes. Finally in step 166 the layer of third photoresist is stripped toremove metal plating covering the third photoresist and form a secondwiring layer. Alternatively, the surface of the substrate may beflattened (planarized) using chemical-mechanical polishing to remove anymetal plating the third photoresist to form the second wiring layer.

A second wiring layer 530 is shown in FIG. 5 which is not an externalwiring layer. In order to produce the structure of FIG. 5 after steps140-166 were all completed (except steps 156 & 158 were not done thefirst time) then the steps were all performed a second time to produce athird wiring layer 532.

In step 170 of FIG. 1(h) a layer of solder resist 533 is formed over theexterior wiring layer 532 as shown in FIG. 5. The solder resist may beapplied as a liquid or paste by roller coating, curtain coating orscreening onto the surface or dry film may be laminated to the surface.Then in step 172 of FIG. 1(h) windows 534 are formed in the solderresist over pads 536 and 537 for surface mount components and lands 538for PIH components. Preferably, the photoresist is a photoimagabledielectric and windows are photoimaged and then the dielectric may becured to make it permanent. For screened solder resist the windows areformed during screening onto the wiring layer. Pads 536 are preferablyspaced 5 to 15 mils apart for connection of a flip chip, or 10 to 30mils for leaded components and pads 537 are spaced at 30 to 50 mils forconnection of a BGA (Ball Grid Array) module.

The circuitized substrate of the invention has improved wirability dueto reduced via diameters and reduced land diameters of the first andsecond wiring layer. In step 180 of FIG. 1(i) joining material 540 (FIG.5) is screened into the windows onto the pads for surface mountconnection. Alternately the joining material may be screened onto thecomponent terminals or the pads or terminals may otherwise be coatedwith joining material. The joining material may be an ECA withconductive particles or a TLP system or a solder paste or a solder alloymay be provided on the pads or terminals and a flux applied to the padsand/or terminals for soldered connection. Solder paste consists ofliquid flux and metal particles which melt during reflow heating to formmolten solder alloy such as eutectic Pb/Sn solder (e.g. Pb and 30-80% Snpreferably 55-70% Sn). In step 182 the terminals (balls, leads, pads) ofsurface mount components are positioned at the pads (close enough forreflowed connection between the pads and the terminals). In step 184 thejoining material is cured. For solder paste the curing includes heatingthe paste above the melting temperature of the solder alloy. In step 186the joining material is cooled to form solid joints between theterminals and pads.

When PIH components are required then steps 190-196 of FIG. 1(j) arealso performed. In step 190 PIH components are placed on the substratewith pins or leads of the component in PTHs. In step 192 flux is appliedinto the holes to provide a more solder wettable metal surface. In step194 the substrate is moved over a wave or fountain of solder in contactwith the molten solder which wets to lands on the bottom of the boardand fills the PTHs by capillary action (surface tension). Then thesolder is cooled to form solid joints of solder alloy.

Alternatively, for PIH components solder paste may be applied to the topsurface of the substrate over the lands around the PTHs and the pins ofthe components inserted through the paste deposits. Then during reflowfor the surface mount components the solder paste reflows to form solderalloy which fills up the respective PTH to connect the PIH components.

FIGS. 2(a)-2(c) illustrate an alternative embodiment for the steps140-166 in FIGS. 1(e)-1(g) of the process for forming additional wiringlayers such as a second wiring layer on each side of the substrate.FIGS. 1(e)-1(g) illustrate an additive process and FIGS. 2(a)-2(c)illustrate a subtractive embodiment.

Steps 200-202 in FIG. 2(a) are similar to steps 156-158 and the abovediscussion thereof applies. Steps 210-230 are similar to steps 160-164in FIG. 1(g) and the above discussion thereof applies. Steps 240-244 aresimilar to steps 150-154 above in FIG. 1(f) and the above discussionapplies. In step 246, exposed copper is etched to form a second wiringlayer interconnected to the first wiring layer by conductive vias. Instep 248, the third layer of photoresist is stripped.

FIG. 6 illustrates computer system 600 of the invention with increasedperformance due to higher component densities and resulting shortersignal flight time. The system includes an enclosure 602 in which apower supply 604 and one or more circuit boards 606, 608, 610 aremounted. The circuit boards communicate through interconnect bus 612.The circuit boards include multiple components including pin grid arraymodule 614, thin small outline package 616, ceramic J-lead component618, ball grid array module 620, quad flat pack 622, flip chip 624,column grid array module 626. The components include one or more CPUs,dynamic RAMS, static RAMS, and I/O processors connected to ports 626,628 for communication with computer peripherals such as keyboards, mice,displays, printers, modems, networks.

Although the invention has been described specifically in terms ofpreferred embodiments, such embodiments are provided only as examples.Those skilled in the art are expected to make numerous changes andsubstitutions, including those discussed above, in arriving at their ownembodiments, without departing from the spirit of the present invention.Thus, the scope of the invention is only limited by the followingclaims.

We claim:
 1. A process for manufacturing circuit boards,comprising:providing a circuitized substrate having an electricallyinsulative surface; laminating a metal foil to said electricallyinsulative surface of said circuitized substrate; thereafter formingfirst holes through said metal foil and extending into said circuitizedsubstrate; substantially filling said first holes with a filler materialincluding an organic base filled with electroconductive particles;thereafter seeding the surface of said filler material; electrolesslyplating to form a coating of copper on said surface of said fillermaterial and on said metal foil; and thereafter etching away portions ofsaid electrolessly plated metal foil to form a wiring layer.
 2. Theprocess of claim 1 wherein said holes are substantially filled with afiller material comprising a cresol novolac-epoxy resin organic basefilled with electroconductive particles.
 3. The process of claim 1wherein said first holes are substantially filled by injecting saidfiller material into said first holes with sufficient heat an pressureto substantially fill said first holes with said filler material.
 4. Theprocess of claim 1 wherein after said filling step, forming second holeshaving walls in said circuitized substrate, seeding said walls of saidsecond holes, and electrolessly plating said walls of said second holes.